Environmental Engineering Reference
In-Depth Information
necessitating higher oxygen coverages prior oxide formation, and higher electrode
potentials to initiate this process.
While we have focused here on the oxygen adsorption at the electrode surface, in
the following, we will discuss the part of the phase diagram shaded gray and labeled
“Oxide formation” in Fig 5.10b.
5.3.5 Pt Oxide Growth
In the Pt bulk oxide range of the phase diagram, which is relevant provided that (5.26)
is fulfilled, the bulk electrode is Pt oxide and no longer pure Pt. Therefore the corre-
sponding term in (5.28) that accounts for the bulk electrode reservoir now has to
involve g
bulk
Pt
x
O
y
. Since the bulk electrode should be in thermodynamic equilibrium
with the surroundings,
xg
bulk
Pt
þ
ym
O
¼
g
bulk
(5
:
30)
Pt
x
O
y
holds [see (5.8)], which allows us to introduce the Gibbs energy of the bulk oxide.
According to (5.29), each oxygen comes from a water molecule of the electrolyte,
which means that m
O
¼
m
H
2
O
þ
2eDf. Insertion of this condition into (5.30) can be
used to obtain a modified interfacial free energy similar to (5.28):
¼
1
A
N
Pt
x
g
00
T, a
H
2
O
, a
Pt
x
O
y
, Df
g
bulk
Pt
x
O
y
GT, a
H
2
O
, a
Pt
x
O
y
, N
O
, N
Pt
T, a
Pt
x
O
y
Dm
H
2
O
(T, a
H
2
O
)
þ
2eDf
þ
y
N
Pt
x
N
O
(5
:
31)
In order to evaluate the structure of the electrode surface under high positive elec-
trode potentials, we used the calculated DF T energies for all different slabs of low
index Pt bulk oxide surfaces [Jacob, 2007b], together with (5.31), to obtain the corre-
sponding (T, a, Df) phase diagrams for the two very stable Pt bulk oxides, a-PtO
2
and
b-PtO
2
. Similar to Fig. 5.10, that was for an oxygen overlayer on a pure Pt electrode,
Fig. 5.11 (Plate 5.2) shows the stable bulk oxide surfaces for different Dm
H
2
O
and Df
conditions. To each plot we again have added the temperature scale that corresponds to
an activity a
H
2
O
¼
1, meaning an ideal solution.
For the a-PtO
2
system, we find that above an electrode potential of 1.2 V, the (001)
surface with bulk composition is most stable and shows only minor relaxation effects
(denoted as (001)-O in Fig. 5.11a). This surface structure corresponds to experimental
UHV measurements of surface oxides on Pt(110), supported by DFT calculations
[Li et al., 2004]. In the case of a very thin surface layer, the layer composition
might even be PtO. Increasing the electrode potential above 2.0 V would cause stron-
ger interactions with the surrounding water dipoles and lead to a a-PtO
2
(011) surface
with an enrichment of oxygen (as O
2-
) on the surface.
For the b-PtO
2
system, we find the same general that increasing the electrode
potential leads to surface structures with more oxygen, although the transition
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